The invention relates to a temperature control system for pulling down the temperature in a conditioned space to a set point temperature, and for maintaining the set point temperature of the conditioned space; and more particularly the invention relates to a hybrid temperature control system that includes a first evaporator coil which utilizes a first heat absorbing fluid to provide primary temperature control of the conditioned space air and a second evaporator coil a distance from the first evaporator coil where the second evaporator coil utilizes a second heat absorbing fluid to provide supplemental temperature control of the conditioned space air.
Mobile temperature control units are typically mounted on one end of trailers, trucks or containers to maintain the cargo transported in the trailer, truck, or container conditioned space at a desired set point temperature during shipment. Known temperature control units may be mechanical units which utilize a hydroflurocarbon-based refrigerant to maintain the conditioned space ambient fluid at the desired set point temperature. As illustrated schematically in FIG. 1, prior art mechanical temperature control unit 10 is generally comprised of a compressor 11 that raises the pressure of a known refrigerant gas, a condenser 12 flow connected to the compressor to condense the high pressure refrigerant gas to a liquid, and an expansion valve 13 for controlling the refrigerant flow to an evaporator 14. The evaporator 14 includes evaporator coils 17 which are enclosed by an evaporator housing 20 having an evaporator inlet 16 through which conditioned space air enters the evaporator and an evaporator discharge 18 through which conditioned space air reenters the conditioned space.
Warm conditioned space air flows into the evaporator inlet 16, continues across the evaporator coils 17 and is discharged through evaporator discharge 18. The refrigerant that flows through the evaporator coils 17 absorbs heat from the conditioned space air, and in this way pulls down the temperature of the conditioned space air to a predetermined set point temperature and thereby maintains the conditioned space at the set point temperature.
In operation, high cooling capacities are needed to pull down a higher temperature conditioned space to the desired lower set point temperature in a relatively short time. After the conditioned space has been pulled down to the desired set point temperature, the cooling capacities required to maintain the conditioned space set point temperature are modest relative to the required pull down cooling capacities.
Conventional mechanical temperature control units provide the required variable cooling capacities by utilizing a compressor prime mover (not shown in FIG. 1) that drives the compressor at high and low speeds to provide high and low cooling capacities. However, even known mechanical temperature control units that utilize multi-speed prime movers cannot provide the cooling capacities required during peak demand periods. For example, during transportation of cargo, the doors to the trailer or truck are typically left open while the cargo is unloaded from the conditioned space. The temperature of the cargo conditioned space increases as the warm outside ambient air flows into the trailer conditioned space. The doors may be left open for an hour or more during unloading. After the delivery has been made and the doors are again closed, the conditioned space is pulled down to reestablish the conditioned space set point temperature. If the temperature of the conditioned space is not pulled down quickly, the load can spoil. Known mechanical units cannot provide the cooling capacity needed to quickly initially pull down the conditioned space or reestablish the set point temperature after cargo unloading.
In order for known mechanical units to achieve the desired pull down capacities, the size of conventional mechanical refrigeration units would need to be increased considerably. However this is not a realistic alternative since such units would be too large to be effectively used in the trailer, truck or container applications and such larger capacity mechanical units would be higher in cost, would be less efficient, would weigh more and would be noisier than conventional mechanical units.
A non-mechanical temperature control unit has been developed to meet the peak cooling demands at initial pull down and during pull down to reestablish the set point temperature in the conditioned space. Such non-mechanical temperature control units utilize a cryogen fluid to produce the desired cooling in the conditioned space. FIG. 2 schematically illustrates a prior art cryogen-based temperature control system 30 that includes a supply of cryogen liquid in cryogen tank 32 and the cryogen may be liquid carbon dioxide LCO.sub.2 for example. An electronic expansion valve 34 or other valve means regulates the supply of cryogen through the evaporator coil 38 of evaporator 36. A microprocessor 37 adjusts the expansion valve position by sending a signal to the valve in response to the sensed temperature at the evaporator unit 36. A vapor motor 40 drives a fan 45 which draws conditioned space air through the evaporator 36 and across the evaporator coil 38. The rotating vapor motor turns alternator 41 which charges a temperature control unit battery (not shown).
The higher temperature conditioned space air is drawn into the evaporator and across coil 38. The cryogen liquid flowing through the evaporator coil absorbs heat from the conditioned space air and the lower temperature air is discharged from the evaporator 36 into the conditioned space in the direction of arrows 43. The cryogen is vaporized as it absorbs heat from the conditioned space air. The cryogen vapor flows out of the evaporator and drives the vapor motor 40. The spent cryogen vapor is exhausted from the vapor motor to atmosphere through exhaust 42 and muffler 39.
The liquid cryogen can provide the cooling capacity required to quickly pull down the conditioned space. However, there are limitations associated with non-mechanical, cryogenic based temperature control units. First, cryogen units are limited by how fast one wants to drop the cargo temperature and by practical considerations so that fresh loads such as produce are not frozen. The supply of cryogen typically only lasts one to three days and when the cryogen supply is exhausted the tank must be refilled. It may be difficult to locate a cryogen filling station. If the cryogen units are to provide a defrost and heating capability, a heating fuel and necessary heating components must be provided.
Hybrid mechanical and non-mechanical temperature control systems have been developed. These systems directly spray a volume of cryogen into the conditioned space during pull down of the conditioned space to the set point temperature. As a result, the conditioned space air is displaced and the conditioned space is comprised primarily of cryogen which is undesirable for most applications. The cryogen is not breathable and can negatively affect some foods.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.